Neue Schichtkonzepte zur Abscheidung von kubischem Bornitrid

Abstract

Boron nitride in the cubic zinc-blende structure (c-BN) has been well known since 1950s for its outstanding physical and chemical properties, such as the second highest hardness and thermal conductivity next to diamond, widest bandgap within III-V semiconductors, chemical inertness against ferrous alloys, and so forth. Such superior characteristics, in particular with respect to their potential applications in modem coating technologies, have invoked considerable interest in the growth of c-BN films by means of various physical vapor deposition (PVD) and plasma-enhanced chemical vapor deposition (PECVD) techniques. Despite intensive investigations carried out in the past two decades, however, practical industrial applications of c-BN films are nowadays still not successfully achieved. The primary difficulty stems from the high compressive stress and poor adhesion of deposited films, which usually lead the films with a moderate thickness of only 300 nm or even below to flake from their substrates. The present study is thus devoted to the development of novel processing concepts in an attempt to get through this technical problem, namely to produce low stress, adhesive and thick c-BN films. Due to its widespread availability in industry as well as its flexible variation of parameters, radio-frequency (rf) magnetron sputtering was adopted in this work as the deposition method. An adequate ion bombardment of the growing film essential for the formation of the cubic boron nitride phase was established through a controlled rf- or dc-bias applied to the substrate electrode. The deposition was operated at various pressures between 0.08 and 0.5 Pa either in a non-reactive mode using pure argon as working gas, or alternatively in a reactive mode with an argon/nitrogen mixture. (100)-oriented single-crystalline silicon wafer and hard metal (Quality Grade P35 after ISO 513) were used as the substrates. To enable an easy upscaling and transfer of laboratory coating concepts for industries as well as for other coating techniques, the kinetic processes for the deposition were further described according to those equipment-independent parameters like ion energy, fluxes of film-forming species, ion fluxes as well as substrate temperature. The phase structure of the deposited films was primarily characterized by Fourier transform infrared spectroscopy (FTIR), and further confirmed by electron energy loss spectroscopy (EELS), X-ray diffraction (XRD) and TEM investigations. The internal stress was estimated according to the curvature radii of the coated silicon beams. In addition, the mass density of the films was evaluated from X-ray reflectivity measurements. The influence of substrate bias and, hence, of ion energy was then discussed within the frame of the subplantation model with respect to the variation of c-BN fraction, internal stress, mass density as well as the phase transformation from the sp 2 -coordinated BN into the cubic phase. Under optimized growth parameters, FTIR measurements indicate an average c-BN fraction of 85% for the deposited films, whereas the residual sp 2 -bonding exists mainly within the initial nucleation region between the substrate and the top c-BN layer. However, such films usually exhibit an exceedingly high compressive stress up to -29 GPa resulted quite clearly from the intensive ion bombardment during deposition.